Forms of Arsenic in Soil
Generally, As concentrations in uncontaminated soils seldom exceed 10 mg kg-1. However, anthropogenic sources of As have elevated the background concentration of As in soils (Adriano 2001). For example, in areas near As mineral deposits, As levels in soils may reach up to 9,300 mg kg-1 (Ashley and Lottermoser 1999).
Depending on the nature of the geogenic and anthropogenic sources, As concentration in soils can range from <1 to 250,000 mg kg—1. However, there is a large fluctuation among countries due to variation in soil parent material, for example, calcareous soils can be expected to have higher levels of As than noncalcareous soils (Ashley and Lottermoser 1999). Arsenic forms solid precipitates with Fe, aluminum (Al), calcium (Ca), magnesium (Mg), and Ni. A number of studies involving solid-phase speciation have shown that As is prevalent mostly in the oxalate fractions associated with amorphous and crystalline Fe and Al oxides, indicating the strong affinity of As for these soil components (Wenzel et al. 2001).
The common valence states of arsenic in nature include —3, 0, +3, and +5 (Leonard, 1991, Jain and Ali 2000). In soils, the most often encountered arsenic forms are inorganic As(III) (arsenite) and As(V) (arsenate) (Cullen and Reimer 1989; Balasoiu et al. 2001). Methylated species, monomethyl arsenic acid (MMAA), dimethyl arsenic acid (DMAA), and trimethyl arsine oxide (TMAO), dominate in biomass, but have also been detected in soils (Leonard 1991). In addition, As(V) and As(III) can be volatilized to arsine (AsH3), MMAA to monomethyl arsine (CH3AsH2, MMA), DMAA to dimethylarsine [(CH3)2AsH, DMA], and TMAO to trimethyl arsine [(CH3)3As, TMA] (Cullen and Reimer 1989). The evolution of arsines is thus greatly dependent on the form of arsenic in soil and most often arsines are formed from methylated species (Gao and Burau 1997). The trivalent compounds are generally more toxic than the pentavalent compounds (Cervantes et al. 1994). The most toxic of them all is arsine gas (AsH3; Leonard 1991). Organic arsenical compounds exist but these are generally nontoxic (Gochfeld 1995).
In solution at neutral pH, arsenic acid exists as the arsenate oxyanion (Rosen 2002). The pKa of arsenous acid is 9.2, so that, at neutral pH, it would be primarily present in solution as neutral As(OH)3. As described later, this difference in pKa is relevant for the type of transport system that catalyzes uptake of the pentavalent and trivalent forms of arsenic. Compared to As(V), As(III) demonstrates greater mobility. The difference in mobility results from the high affinity of As(V) for Al, Fe, and Mn oxides (Cullen and Reimer 1989). The other aspect of arsenic chemistry relevant to biological activity is reactivity of As(III) as a soft metal ion, forming strong bonds with functional groups such as the thiolates of cysteine residues and the imidazolium nitrogens of histidine residues. The relative proportions of these oxidation states in a given environment depend on the biological processes involved as well as on the local physicochemical conditions, including the redox potential (Eh) and the hydrogen potential (pH), which are important factors. Since the pKa contents of arsenate (H3AsO4) are pKa1 = 2.19, pKa2 = 6.94 and pKa3 = 11.5, the H2AsO4 form predominates in oxidative media with pH levels below 6.9, whereas the HAsO4 2— form predominates at higher pH levels. In the case of arsenite, the lowest pKa levels recorded are equal to 9.22. In most natural waters with pH levels below 9.2 as well as in slightly reductive environments, As(OH)3 is the main form present (Inskeep et al. 2002). The solubility of arsenic and the resulting bioavailability of this element are closely related to its speciation. Several studies have shown that the reduction of arsenate into arsenite results in the solubilization of this element. However, arsenate may be sequestered after being co-precipitated with ferric iron (Foster 2003) or sulfur (O'Day et al. 2004) or adsorbed by clay, calcite, organic matter, or hydroxides, in particular ferric oxyhydroxides (Morin et al. 2003).
Continue reading here: Arsenic Toxicity of Food Chain
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